Electrical power is transmitted from substations through cables connected to electrical equipment or other cables which, in turn, connect to other pieces of electrical apparatus. The cables can be terminated on bushings which may pass through walls of metal-encased equipment such as capacitors, transformers or switchgear. The bushings also can connect two cables together.
The bushings typically are made from insulating materials such as epoxy, other plastics and various types of rubber. The construction of the bushing uses multiple layers. There is typically a conductor made from a metal, such as copper or aluminum, that efficiently conducts electrical current. A voltage shield, made of a conductive material, covers an interior of the bushing and surrounds the conductor. The voltage shield causes air within the bushing or around the conductor to be at the same electrical potential as the conductor so as to inhibit discharges that could damage the bushing. An insulating layer is molded over the voltage shield to insulate the bushing from the outside environment. An external ground shield, made of a conductive material, is molded around the outside of the insulating layer to maintain the exterior of the bushing at ground potential. This allows any capacitive charge that develops from the electric field and voltage drop across the insulation to be drained away, which increases safety by preventing capacitive accumulation of charge on the outer diameter of the bushing.
When installing or repairing power cables, it is desirable to create a break in the circuit that can be seen by the operator. One way that this is done is by removing a cable from the bushing and grounding the cable at its connection point. This requires unbolting and removing a connector from the bushing with remote operating tools that keep the operator several feet away from the bushing and may be difficult to operate. Another way this is done is to place a switch in the circuit that has contacts that open to provide a gap and provisions to allow the line operator to see the gap, before applying ground to the end of the cable. Such switching devices often use transparent liquids, such as oils, or transparent gases, such as air or SF6. A third system provides the ability to ground the circuit, but without a visible disconnection.